Alignment of carbon nanotubes comprising magnetically sensitive metal oxides in nanofluids

ABSTRACT

The present invention is a nanoparticle mixture or suspension or nanofluid comprising nonmagnetically sensitive nanoparticles, magnetically sensitive nanoparticles, and surfactant(s). The present invention also relates to methods of preparing and using the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of U.S. Ser. No.13/234,144 filed Sep. 15, 2011, which claims priority under 35 U.S.C.§119 to provisional application Ser. No. 61/383,670 filed Sep. 16, 2010,both of which are herein incorporated by reference in their entirety.

FEDERALLY SPONSORED RESEARCH

United States Army Research Laboratories, Cooperative AgreementW911NF-08-2-022. National Aeronautics and Space Administration (NASA)EPSCoR Award No. NNX09AU83A. Consequently the U.S. Government may havecertain rights in this invention.

TECHNICAL FIELD

The present invention relates to nanoparticle mixtures or suspensions.More specifically it relates to compositions and methods of makingnanofluids for various applications, including applications related topolymers, heat transfer and conductivity, including electrical andthermal conductivity.

BACKGROUND OF THE INVENTION

Nanoparticles, such as carbon nanoparticles, and nanofluids, or fluidscontaining nanoparticles, have broad industrial application, includinguse in polymers, liquid polymers/polymer fluids, polymer dispersions,liquid resins, films, coatings films, reinforced polymer composites,transparent electrodes for displays and solar cells, electromagneticinterference shielding, sensors, medical devices and pharmaceutical drugdelivery devices. For example, in the field of semiconductors andelectronic devices, nanoparticles, and specifically, conductivenanoparticles of carbon, metals and the like, have been known andenabled to the industry for many years. Examples of US patentdisclosures of such particles and processes are provided, by way ofnon-limiting examples, in U.S. Pat. Nos. 7,078,276; 7,033,416;6,878,184; 6,833,019; 6,585,796; 6,572,673; 6,372,077. Also, theadvantages of having ordered nanoparticles in these applications is wellestablished. (See, for example, U.S. Pat. No. 7,790,560). By way ofanother example, the combination of nanoparticles and liquid polymershave been found to improve important properties of rubber articles, suchas vehicle tires, and in particular, the tread portion of vehicle tires.U.S. Pat. No. 7,829,624.

Nanofluids have also been used extensively in heat transfer fluids, andprovide many advantages over prior heat transfer fluids, includingthermal conductivities far above those of traditional solid/liquidsuspensions, a nonlinear relationship between thermal conductivity andconcentration, strongly temperature-dependent thermal conductivity, anda significant increase in critical heat flux. In addition, althoughconventional heat transfer fluids, such as water, mineral oil, andethylene glycol play an important role in many industries includingpower generation, chemical production, air conditioning, transportation,and microelectronics, their inherently low thermal conductivities havehampered the development of energy-efficient heat transfer fluids thatare required in a plethora of heat transfer applications. It has beendemonstrated recently that the heat transfer properties of theseconventional fluids can be significantly enhanced by dispersingnanometer-sized solid particle and fibers (i.e., nanoparticles) influids (Eastman, et al., Appl. Phys. Lett. 2001, 78(6), 718; Choi, etal., Appl. Phys. Lett. 2001, 79(14), 2252).). However, there arelimitations for these nanofluids as well. For example, in a typicalnanofluid, individual nanoparticles, such as carbon nanotubes (CNTs),are irregularly positioned in the nanofluid with only a random andinfrequent chance for them to be in contact with each other, and onlyvery high concentrations (e.g., 1 vol % (˜1.4 wt %) of nanoparticle(such as SWNT)) of these nanoparticles seem to produce any noticeableincrease in the effective thermal conductivity (Kim, et al., J.Thermophys. Heat Transfer 21 (2007) 451-459; Xie, et al., J. Appl. Phys.94 (2003) 4967-4971; Hong, et al., J. Thermophys. Heat Transfer 21(2007) 234-236; Wamkam, et al., J. Appl. Phys. 109 (2011)024305-024310). However, at these high concentrations, the nanofluid isvery viscous and becomes “mud-like,” which makes it much less useful asa coolant or for lubrication applications.

The observed substantial increases in the thermal conductivities ofnanofluids can have broad industrial applications and can alsopotentially generate numerous economical and environmental benefits.Enhancement in the heat transfer ability could translate into highenergy efficiency, better performance, and low operating costs. The needfor maintenance and repair can also be minimized by developing ananofluid with a better wear and load-carrying capacity. Consequently,classical heat dissipating systems widely used today can become smallerand lighter, thus resulting in better fuel efficiency, less emission,and a cleaner environment.

Recently, increased thermal conductivity has been associated withexposing fluids with iron oxide-encapsulated nanotubes to a magneticfield. The theory behind this approach is that the magnetic field alignsthe iron-oxide encapsulated nanotubes, which results in improved thermalconductivity. Although promising, limitations and unknowns were alsorevealed. For example, the improved thermal conductivity was found to besporadic and not observed in every single instance.

Accordingly, there is a great need for the development of nanoparticlemixtures or suspensions and nanofluids that have or result in enhancedproperties.

BRIEF SUMMARY OF THE INVENTION

An objective of the present invention is to provide a nanoparticlemixture, suspension or fluid that when exposed or subjected to amagnetic field results in a mixture, suspension or fluid with alignednanoparticles. Such a composition is believed to have utility for manyapplications in different industries.

Although not wishing to be bound by any particular scientific theory, itis believed aligned nanoparticles, especially carbon nanoparticles,provide various benefits over other nanofluids and other fluids, byreducing the amount of carbon chain interaction, improving the flow ofions, and providing a more ordered structure. In the instance ofpolymers and polymer liquids, fluids, dispersions, oils, suspensions andmixtures, the alignment is believed to prevent or help prevent theaggregation of the nanoparticles and lead to enhanced polymercharacteristics. These enhanced polymer characteristics includereductions in scission and degradation, improved conductivity (e.g.,electrical, energy, heat, etc.), enhanced chemical properties (throughmore ordered spatial orientation that results in more consistentintramolecular forces and dipole interaction), physical properties(e.g., a more ordered spatial orientation imparts increased structuralflexibility and strength). As for heat transfer applications, thisalignment is believed to provide enhanced thermal conductivityproperties.

Generally, the present invention relates to compositions of nanoparticlemixtures or suspensions and nanofluids, including hydrophilicnanofluids, nanolubricants and nanogreases. The nanoparticle mixture orsuspension of the present invention comprises magnetically sensitivenanoparticles, nonmagnetically sensitive nanoparticles andsurfactant(s). The nanofluid comprises the nanoparticle mixture orsuspension and a fluid (or liquid). Other useful components, such aschemical additives, may be added to the nanoparticle mixture orsuspension or nanofluid as well. The magnetic nanoparticles, nonmagneticnanoparticles, surfactant, and/or any other components may be dispersedin the nanofluid as separate components or in combinations (in anyorder). Once dispersed in the thermal transfer fluid, the nanoparticlesand/or magnetically sensitive materials or metal oxides are exposed orsubjected to a magnetic field, which produces a nanofluid with improvedpolymer and thermal conductivity characteristics.

In one aspect, surfactant(s) are attached to the nonmagneticallysensitive particles forming a surfactant and nonmagnetically sensitiveparticle complexes (S/NSP Complexes). The S/NSP Complexes are then, inturn attached to the magnetically sensitive nanoparticles. In oneembodiment, the attachment occurs prior to dispersement of thesurfactant, nonmagnetically sensitive particles, and magneticallysensitive particles in the fluid. In another embodiment, the attachmentoccurs after dispersement of the surfactant(s), nonmagneticallysensitive particles, and magnetically sensitive particles in the fluid.In yet another embodiment, the nanoparticles and surfactant(s) areattached to each other by electrostatic attraction.

In one aspect, the nanoparticles of the nanofluids of the presentinvention are nanotubes. In one embodiment, the nanotubes single-wallednanotubes, double-walled nanotubes or multi-walled nanotubes. In apreferred embodiment, the nanotubes are single-walled nanotubes (SWNT),multi-walled nanotubes (MWNT) or double-walled nanotubes (DWNT).

In a second aspect, the nonmagnetically sensitive nanoparticles of thepresent invention are various materials that have been used to makepolymers and heat transfer nanofluids. In one embodiment, thenonmagnetically sensitive nanoparticles are carbon, graphite or grapheneparticles. In a particularly preferred embodiment, the nonmagneticallysensitive nanoparticles are carbon particles, and more preferably,carbon nanotubes. In another embodiment, a substantial amount, orgreater than about 90%, of the nonmagnetically sensitive nanoparticlesare aligned. In a preferred embodiment, greater than or about 95% of thenonmagnetically sensitive nanoparticles are aligned.

In a third aspect, the magnetically sensitive nanoparticles of thepresent invention are various materials that respond orientationally toa magnetic or electric field. In an embodiment, the magneticallysensitive nanoparticles are magnetically sensitive metals or metaloxides. In a preferred embodiment, the magnetically sensitivenanoparticles are: Fe, Co, Fe₂O₃, or Fe₃O₄. In a particularly preferredembodiment, the magnetically sensitive nanoparticles are Fe₂O₃.

In a fourth aspect, the surfactants of the nanofluids of the presentinvention are ionic or charged surfactants, but selected to “match” thecharge of the magnetically sensitive nanoparticles. By way of example,in one embodiment, if the magnetically sensitive nanoparticle has apositive charge, a surfactant with a net negative charge should beselected. In another embodiment, if the magnetically sensitivenanoparticle has a negative charge, a surfactant with a net positivecharge should be selected. Accordingly, the pH of the surfactant shouldbe considered when selecting surfactant(s) to be included in thenanoparticle mixtures or suspensions or nanofluids of the presentinvention. Therefore, in an embodiment, the surfactant(s) of the presentinvention have an appropriate pH that maintains, imparts (or helps toimpart) or results in a desired charge effect or net charge, whetherpositive or negative. In a second embodiment, the surfactants areanionic or with a negative net charge. In a preferred embodiment, theanionic surfactants of the present invention are sodium dodecylbenzenesulfonate (NaDDBS). In another embodiment, the surfactants are cationicor with a positive net charge. In a preferred embodiment, the cationicsurfactants of the present invention are cetyl trimethylammonium bromide(CTAB). CTAB is also known as hexadecyl trimethyl ammonium bromide.

In another aspect, the nanofluids of the present invention havecombinations of specific pH ranges and surfactant(s). In one embodiment,if the surfactant(s) have a net negative charge the pH of the fluid isgreater than about 5. In another embodiment, if the surfactant(s) have anet positive charge, the pH of the fluid is less than about 9.

In yet another aspect, it was found that the fluids of the presentinvention having higher dipole moments result in more rapid alignment.Therefore, in one embodiment, the fluids of the present invention have adipole moment at least or greater than about zero (0), at least orgreater than about 1, at least or greater than about two 2, or at leastor greater than about 3.

In accordance with the present invention, a process for preparing ananofluid of the present invention is disclosed. In one aspect, thenonmagnetically sensitive materials, magnetically sensitivenanoparticles and surfactant(s) either as separate components (in anyorder, either singly, in combination or as a mixture or suspension) areadded to or dispersed in a fluid. Other components, such as additives,may be added as well. Then, a magnetic field is applied to or directedon fluid and its components. In one embodiment, the magnetic field isapplied until the alignment of the nonmagnetically sensitivenanoparticles reaches a maximum amount or when the enhancement ofpolymer characteristics or increase of conductivity (such as electricalor thermal conductivity) reaches a maximum.

In one embodiment, the nonmagnetically sensitive particles are attachedto surfactant(s) to form one or more surfactant and nonmagneticallysensitive particle complexes (S/NSP Complexes) prior to dispersing thesurfactants and nonmagnetically sensitive nanoparticles in the fluid. Inanother embodiment, magnetically sensitive particles are attached to theS/NSP Complexes prior to dispersing the surfactants, nonmagneticallysensitive nanoparticles and magnetically sensitive nanoparticles in thefluid. In a separate embodiment, nonmagnetically sensitive particles areattached to surfactant(s) to form one or more surfactant andnonmagnetically sensitive particle complexes (S/NSP Complexes) afterdispersing the surfactants and nonmagnetically sensitive particles inthe fluid. In a further embodiment, magnetically sensitive particles areattached to S/NSP Complexes prior to dispersing the surfactants,nonmagnetically sensitive nanoparticles and magnetically sensitivenanoparticles in the fluid. In a yet another embodiment, electrostaticattraction is used to attach the S/NSP Complexes to the magneticallysensitive particles.

In accordance with the present invention, a method of increasing thealignment of nanoparticles in a fluid (and, thereby, increasing thermalconductivity or enhancing the polymer characteristics of the nanofluid).In one aspect, the method is directed to exposing a nanofluid containingnanoparticles to a magnetic field. In one embodiment, the methodincludes the steps of analyzing or verifying the components of thenanofluid prior to or after exposing the nanofluid to a magnetic field.In a second embodiment, the method includes the step of addingnonmagnetically sensitive nanoparticles, surfactants with theappropriate charge, and/or magnetically sensitive nanoparticles (asseparate components or combinations (such as the S/NSP Complex), if notalready present or in low amounts, to the fluid prior to exposing thefluid to a magnetic field.

Another objective of the present invention is to increase theconductivity of nanofluids with low nanoparticle concentrations.Although conductivity, such as thermal conductivity, generally increaseswith higher nanoparticle concentrations, high concentrations result inhigher viscosity, which may not be desirable for applications in whichlow viscosity is desirable, such as for coolants, An advantage if thenanofluids of the present invention is that increased conductivity, suchas thermal conductivity (TC), is observed even with nanofluids withnanoparticle concentrations at lower levels. In one embodiment, theconcentration of nanoparticles in the nanofluids of the preset inventionis no greater than about 30%, no greater than about 15%, no greater thanabout 10%, no greater than about 5%, no greater than about 2.5%, or nogreater than about 1%, no greater than about 0.5%, no greater than about0.2%, no greater than about 0.1%, or no greater than about 0.05% byweight of nanoparticles

Other aspects of the present invention are described throughout thespecification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows the arrangement of carbon nanotubes in 0.017 weight %SWNT, 0.017 weight % Fe₂O, and 0.17% NaDBBS in DI water.

FIG. 1B shows the arrangement of carbon nanotubes in 0.017 weight %SWNT, 0.017 weight % Fe₂O, and 0.17% NaDBBS in DI water thirty (30)seconds after being exposed to a magnetic field.

FIG. 1C shows the arrangement of carbon nanotubes in 0.017 weight %SWNT, 0.017 weight % Fe₂O, and 0.17% NaDBBS in DI water sixty (60)seconds after being exposed to a magnetic field.

FIG. 1D shows the arrangement of carbon nanotubes in 0.017 weight %SWNT, 0.017 weight % Fe₂O, and 0.17% NaDBBS in DI water one hundred andtwenty (120) seconds after being exposed to a magnetic field.

FIG. 2A shows images of 0.017 weight % SWNT, 0.017 weight % Fe₂O, and0.17% NaDBBS in DI water prior to exposure to a magnetic field.

FIG. 2B shows images of 0.017 weight % SWNT, 0.017 weight % Fe₂O, and0.17% NaDBBS in DI water after exposure to a magnetic field.

FIG. 3 shows changes in the carbon nanotubes in 0.017 weight % SWNT,0.017 weight % Fe₂O, and 0.17% NaDBBS in DI water after exposure to amagnetic field.

FIG. 4 shows the macrogeometrical effect of magnetically aligned 0.017weight % SWNT, 0.017 weight % Fe₂O, and 0.17% NaDBBS in DI water;

FIG. 5A shows magnetically aligned 0.017 weight % SWNT, 0.017 weight %Fe₂O, and 0.17% NaDBBS in hexane.

FIG. 5B shows magnetically aligned 0.017 weight % SWNT, 0.017 weight %Fe₂O, and 0.17% NaDBBS in DI water.

FIG. 5C shows magnetically aligned 0.017 weight % SWNT, 0.017 weight %Fe₂O, and 0.17% NaDBBS in DMF.

FIG. 6A shows the alignment of nanotubes in the direction of a magneticfield in scale bar 100 μm.

FIG. 6B shows the alignment of nanotubes in the direction of a magneticfield in scale bar 10 μm.

FIG. 7A shows the aggregation among carbon nanotubes, surfactant andmetal oxide in 0.017 weight % SWNT, 0.017 weight % Fe₂O, and 0.17% CTABin DI water.

FIG. 7B shows the aggregation among carbon nanotubes, surfactant andmetal oxide in 0.017 weight % SWNT, 0.017 weight % Fe₂O, and 0.17% CTABin DI water thirty (30) seconds after exposure to a magnetic field.

FIG. 7C shows the aggregation among carbon nanotubes, surfactant andmetal oxide in 0.017 weight % SWNT, 0.017 weight % Fe₂O, and 0.17% CTABin DI water sixty (60) seconds after exposure to a magnetic field.

FIG. 7D shows the aggregation among carbon nanotubes, surfactant andmetal oxide in 0.017 weight % SWNT, 0.017 weight % Fe₂O, and 0.17% CTABin DI water one hundred and twenty (120) seconds after exposure to amagnetic field.

FIG. 8A shows the interaction of nanotube, metal oxide and surfactant in0.017 weight % SWNT, 0.017 weight % Fe₂O, and 0.17% CTAB in DI water.

FIG. 8B shows the interaction of nanotube, metal oxide and surfactant in0.017 weight % SWNT, 0.017 weight % Fe₂O, and 0.17% CTAB in DI water onehundred and twenty (120) seconds after exposure to a magnetic field.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to methods of enhancing the properties orcharacteristics of magnetically sensitive nanofluids and compositions ofmagnetically sensitive nanoparticle mixtures and nanofluids, includinghydrophilic nanofluids, nanolubricants and nanogreases. In general, thenanoparticle mixtures and nanofluids of the present invention comprisenonmagnetically sensitive nanoparticles, magnetically sensitivenanoparticles and surfactant(s). Additionally, the nanomixtures andnanofluids of the present invention may further comprise chemicaladditives to provide other desired chemical and physicalcharacteristics, such as antiwear, corrosion protection and thermaloxidative properties.

The present invention also relates to the exposure of the nanoparticlemixtures or suspensions or nanofluids of the present invention to amagnetic field. While not wishing to be bound by any particularscientific theory, it is believed that the electrostatic attractionbetween a surfactant and nonmagnetically sensitive nanoparticle (S/NSP)complex and magnetically sensitive nanoparticle aids in the attachmentand aggregation between and among the N/NSP complexes and magneticallysensitive nanoparticles. Accordingly, it is further believed thatsubjecting nanofluids containing nonmagnetically sensitive nanoparticlesand surfactant(s) dispersed with magnetically sensitive nanoparticles,to a magnetic field aligns, aggregates and/or orients (or helps thealigning, aggregation and/or orienting) of the nonmagnetic nanoparticlesin the nanofluids, which among other things, results in increasedthermal conductivity. In this manner, the nanoparticles become“magnetically guided nanoparticles.”

As used in this disclosure, the singular forms “a”, “an”, and “the” mayrefer to plural articles unless specifically stated otherwise. Tofacilitate understanding of the invention set forth in the disclosurethat follows, a number of terms are defined below.

All patents, patent applications, articles and website information (asof the access date) cited and referenced herein are incorporated byreference in their entireties.

DEFINITIONS

The term “dipole moment” or “electrical dipole moment” refers to ameasure of the separation of positive and negative electrical charges ina system of charges, that is, a measure of the charge system's overallpolarity (with, for example, SI units of Coulomb-meter (C m)).

The term “nanotube” refers to a class of macromolecules which have ashape of a long thin cylinder.

The term “SWNT” refers to a single-walled carbon nanotube.

The term “MWNT” refers to a multi-walled carbon nanotube.

The term “D-SWNT” refers to a double-walled carbon nanotube.

The term “F-SWNT” refers to a fluorinated SWNT.

The term “carbon nanoparticle” refers to a nanoparticle which containprimarily carbon element, including diamond, graphite, fullerenes,carbon nanotubes, carbon fibers, and combinations thereof.

The term “magnetic field” refers to a field of force associated withchanging electric fields, as when electric charges are in motion.Magnetic fields exert deflective forces on moving electric charges.

The term “magnetically sensitive” or “magnetic-field-sensitive” refersto the characteristic of responding orientationally to the presence ofan electric or a magnetic field. The terms “magnetically sensitive” and“magnetic-field-sensitive” are used interchangeably in the presentinvention.

The term “nanoparticle” refers to a particle having at least onedimension that is no greater than about 500 nm, and sometimes no greaterthan about 100 nm, and includes, for example, “nanospheres,” “nanorods,”“nanocups,” “nanowires,” “nanoclusters,” “nanolayers,” “nanotubes,”“nano crystals,” “nanobeads,” “nanobelts,” and “nanodisks.”

The term “nanoscale” refers to a dimension that is no greater than about500 nm, and sometimes no greater than about 100 nm. The terms “nanoscaleparticle” and “nanoparticle” are used interchangeably in the presentinvention.

The term “nonmagnetically sensitive” or “nonmagnetic field sensitive”refers to the characteristic of not responding (or responding weakly)orientationally to the presence of an electric or a magnetic field. Theterms “nonmagnetically sensitive” and “nonmagnetic field sensitive” areused interchangeably in the present invention.

The term “PAO” refers to polyalphaolefin.

The term “Polyol ester” refers to an ester of an organic compoundcontaining at least two hydroxyls with at least one carboxylic acid.

The term “surfactant” refers to a molecule having surface activity,including wetting agents, dispersants, emulsifiers, detergents, andfoaming agents, etc.

Nanoparticles:

The nanoparticles of the present invention may be any conventionalnanoparticle used in polymers, polymer fluids, and thermal transferfluids. The nanoparticles may be selected based upon their stability,solubility, thermophysical, electrical, mechanical, size, and zetapotential (e.g., surface charge) properties.

The magnetically sensitive nanoparticles include material which respondsorientationally to the presence of an electric or a magnetic field, suchas magnetically sensitive metals and metal oxides. Such magneticallysensitive metals and metal oxides include, but are not limited to, Fe,Co, Fe₂O₃, and Fe₃O₄. The magnetic nanoparticles may be paramagnetic orferromagnetic.

As for nonmagnetically sensitive particles, preferred nonmagneticallysensitive nanoparticles of the present invention are carbonnanoparticles, and particularly preferred nonmagnetically sensitivenanoparticles are carbon nanotubes. A more detailed discussion of carbonnanoparticles and carbon nanotubes is set forth supra.

In an embodiment, two or more nanoparticles are attached to each other.In one preferred embodiment, carbon nanoparticles, such as carbonnanotubes, are attached to metals or metal oxides. Any conventionalmethod may be used to attach the nanoparticles to each other. However,it has been observed that carbon nanotubes and iron oxide (Fe₂O₃)dispersed together in a deionized water/ethylene glycol solution to forma nanofluid and then, exposed to a magnetic field do not result in anyincreased thermal conductivity for the nanofluid. While not wishing tobe bound by any scientific theory, it is believed that metal or metaloxide may detach from the nanotube under a strong magnetic field or thatthe amount of metal or metal oxide that was attached to the nanotube wastoo trivial. Therefore, a preferred embodiment is to use a method thatcan create a binding force that can withstand the shear forces of astrong magnetic field, such as electrostatic attraction, to attach thenanoparticles to each other. In this regard, selecting a surfactant to“match” the charge of the magnetically charged nanoparticle is importantfor attaching the nonmagnetically charged nanoparticles to themagnetically charged nanoparticles. For example, if the magneticallysensitive nanoparticle has a positive charge, a surfactant with a netnegative charge should be selected so as to aid in the connecting thenonmagnetically sensitive nanoparticle, via the S/NSP Complex, to themagnetically sensitive nanoparticle (and enhance the electrostaticattraction between the nanoparticles). In one embodiment, if thesurfactant(s) have a net negative charge the pH of the fluid is greaterthan about 5. In another embodiment, if the surfactant(s) have a netpositive charge, the pH of the fluid is less than about 10. In addition,by providing a fluid having an appropriate pH, a charge effect betweenthe surfactant molecules and the magnetically sensitive nanoparticlescan be maintained. The nonmagnetically sensitive nanoparticles can thenbe maintained in suspension due to the charge effect between the headgroups on the surfactant molecules. Therefore, in another aspect, thenanofluids of the present invention have combinations of specific pHranges and surfactant(s). In one embodiment, if the surfactant(s) have anet negative charge the pH of the fluid is greater than about 5. Inanother embodiment, if the surfactant(s) have a net positive charge, thepH of the fluid is less than about 9. As an alternative embodiment, thepH of the fluid may be adjusted below the pH point of zero charge, or“pHpzc” at which pH the magnetically sensitive nanoparticle's surface isneutral.

Carbon Nanoparticles:

Carbon nanoparticles have a high heat transfer coefficient and highthermal conductivity, which often exceed these of the best metallicmaterial. For example, it has been reported that single wall carbonnanotubes (SWNT) may exhibit a thermal conductivity value as high as2000-6000 W/m-K under ideal circumstances. By contrast, typical heattransfer fluids like water and oil, have thermal conductivity values ofonly 0.6 W/m-K and 0.2 W/m-K, respectively. Many forms of carbonnanoparticles can be used in the present invention, including carbonnanotubes, diamond, fullerenes, graphite, carbon fibers, andcombinations thereof.

Carbon nanotubes (“CNT”) are macromolecules in the shape of a long thincylinder often with a diameter in few nanometers. The basic structuralelement in a carbon nanotube is a hexagon which is the same as thatfound in graphite. Based on the orientation of the tube axis withrespect to the hexagonal lattice, a carbon nanotube can have threedifferent configurations: armchair, zigzag, and chiral (also known asspiral). In armchair configuration, the tube axis is perpendicular totwo of six carbon-carbon bonds of the hexagonal lattice. In zigzagconfiguration, the tube axis is parallel to two of six carbon-carbonbonds of the hexagonal lattice. Both these two configurations areachiral. In chiral configuration, the tube axis forms an angle otherthan 90 or 180 degrees with any of six carbon-carbon bonds of thehexagonal lattice. Nanotubes of these configurations often exhibitdifferent physical and chemical properties. For example, an armchairnanotube is always metallic whereas a zigzag nanotube can be metallic orsemiconductive depending on the diameter of the nanotube. All threedifferent nanotubes are expected to be very good thermal conductorsalong the tube axis, exhibiting a property known as “ballisticconduction,” but good insulators laterally to the tube axis.

In addition to the common hexagonal structure, the cylinder of a carbonnanotube molecule can also contain other size rings, such as pentagonand heptagon. Replacement of some regular hexagons with pentagons and/orheptagons can cause cylinders to bend, twist, or change diameter, andthus lead to some interesting structures such as “Y-,” “T-,” and“X-junctions,” and different chemical activities. Those variousstructural variations and configurations can be found in both SWNT andMWNT. However, the present invention is not limited by any particularconfiguration and structural variation. The carbon nanotube used in thepresent invention can be in the configuration of armchair, zigzag,chiral, or combinations thereof. The nanotube can also containstructural elements other than hexagon, such as pentagon, heptagon,octagon, or combinations thereof.

Another structural variation for MWNT molecules is the arrangement ofthe multiple tubes. A perfect MWNT is like a stack of graphene sheetsrolled up into concentric cylinders with each wall parallel to thecentral axis. However, the tubes can also be arranged so that an anglebetween the graphite basal planes and the tube axis is formed. Such MWNTis known as a stacked cone, Chevron, bamboo, ice cream cone, or piledcone structures. A stacked cone MWNT can reach a diameter of about 100nm. In spite of these structural variations, all MWNTs are suitable forthe present invention as long as they have an excellent thermalconductivity. The term MWNT used herein also includes double-wallednanotubes (“DWNT”).

Carbon nanotubes used in the present invention can also encapsulateother elements and/or molecules within their enclosed tubularstructures. Such elements include Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y,Zr, Mo, Ta, Au, Th, La, Ce, Pr, Nb, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Mo,Pd, Sn, and W. Such molecules include alloys of these elements such asalloys of Cobalt with S, Br, Pb, Pt, Y, Cu, B, and Mg, and compoundssuch as the carbides (i.e. TiC, MoC, etc.) The present of theseelements, alloys and compounds within the core structure of fullerenesand nanotubes can enhance the thermal conductivity of thesenanoparticles which then translates to a higher thermal conductivenanofluid when these nanoparticles are suspend in a heat transfer fluid.

Carbon nanoparticles used in the present invention can also bechemically modified and functionalized, such as covalently attachedhydrophilic groups to increase their solubility in hydrophilic fluids orlipophilic chains to increase their solubility in hydrophobic oils.Covalent functionalization of carbon nanoparticles, especially carbonnanotubes and fullerenes, has commonly been accomplished by threedifferent approaches, namely, thermally activated chemistry,electrochemical modification, and photochemical functionalization. Themost common methods of thermally activated chemical functionalizationare addition reactions on the sidewalls. For example, the extensivetreatment of a nanotube with concentrated nitric and sulfuric acidsleads to the oxidative opening of the tube caps as well as the formationof holes in the sidewalls and thus produces a nanotube decorated withcarboxyl groups, which can be further modified through the creation ofamide and ester bonds to generate a vast variety of functional groups.The nanotube molecule can also be modified through addition reactionswith various chemical reagents such halogens and ozone. Unlike thermallycontrolled modification, electrochemical modification of nanotubes canbe carried out in more selective and controlled manner. Interestingly, aSWNT can be selectively modified or functionalized either on thecylinder sidewall or the optional end caps. These two distinctstructural moieties often display different chemical and physicalcharacteristics. The functional groups on functionalized carbonnanoparticles may be attached directly to the carbon atoms of a carbonnanoparticle or via chemical linkers, such as alkylene or arylenegroups. To increase hydrophilicity, carbon nanoparticles can befunctionalized with one or more hydrophilic functional groups, such as,sulfonate, carboxyl, hydroxyl, amino, amide, urea, carbamate, urethane,and phosphate. To increase hydrophobicity, carbon nanoparticles may befunctionalized with one or more hydrophobic alkyl or aryl groups. Thefunctionalized carbon particle may have no less than about 2, no lessthan about 5, no less than about 10, no less than about 20, or no lessthan about 50 functional groups on average.

The term “carbon nanotube” used herein refers to all structuralvariations and modification of SWNT and MWNT discussed hereinabove,including configurations, structural defeats and variations, tubearrangements, chemical modification and functionalization, andencapsulation.

Carbon nanotubes are commercially available from a variety of sources.Single-walled carbon nanotubes can be obtained from Carbolex (Broomall,Pa.), MER Corporation (Tucson, Ariz.), and Carbon NanotechnologiesIncorporation (“CNI”, Houston, Tex.). Multi-walled carbon nanotubes canbe obtained from MER Corporation (Tucson, Ariz.) and Helix materialsolution (Richardson, Tex.). However, the present invention is notlimited by the source of carbon nanotubes. In addition, manypublications are available with sufficient information to allow one tomanufacture nanotubes with desired structures and properties. The mostcommon techniques are arc discharge, laser ablation, chemical vapordeposition, and flame synthesis. In general, the chemical vapordeposition has shown the most promise in being able to produce largerquantities of nanotubes at lower cost. This is usually done by reactinga carbon-containing gas, such as acetylene, ethylene, ethanol, etc.,with a metal catalyst particle, such as cobalt, nickel, or ion, attemperatures above 600° C.

The selection of a particular carbon nanoparticle depends on a number offactors. The most important one is that the nanoparticle has to becompatible with an already existing base fluid (a thermal transferfluid) discussed thereafter. Other factors include heat transferproperties, cost effectiveness, solubility, dispersion and settlingcharacteristics. In one embodiment of the present invention, the carbonnanoparticles selected contain predominantly single-walled nanotubes. Inanother embodiment, the carbon nanoparticles selected containpredominantly multi-walled nanotubes. In yet another embodiment, thecarbon nanoparticles are functionalized chemically. The functionalizedcarbon nanoparticles may be soluble in a hydrophilic thermal transferfluid, which are suitable for preparing a hydrophilic nanofluid, or in ahydrophobic thermal transfer fluid, which are suitable for preparing ahydrophobic nanofluid.

Fluid:

In the present invention, the major component of the nanofluid is afluid, which may be either hydrophilic or hydrophobic. The fluid may beany conventional fluid used in polymer and thermal transferapplications. For example, a hydrophilic fluid is commonly used incoolants whereas a hydrophobic fluid is commonly used in a lubricant orgrease.

The fluid may be a single component or multi-component mixture. Forexample, a hydrophilic fluid may contain water, ethylene glycol, anddiethylene glycol in various proportions. The hydrophilic fluid maycontain about 0.1 to about 99.9% by volume of water, about 0.1 to 99.9%by volume of ethylene glycol, and about 0.1 to 99.9% by volume ofdiethylene glycol; and about 20 to about 80%, about 40 to about 60%, orabout 50% by volume of water or ethylene glycol. Typically, diethyleneglycol constitutes a minor component of the hydrophilic fluid, in nogreater than about 20%, no greater than about 10%, or no greater thanabout 5% of the total volume. Nevertheless, the total amount of all thecomponents in a fluid together equals to 100%.

It was found that the fluids of the present invention having higherdipole moments result in more rapid alignment. Therefore, in oneembodiment, the fluids of the present invention have a dipole moment atleast or greater than about zero (0), at least or greater than about one(1), greater than or about two (2), greater than or about (3). Examplesof fluids for use in the present invention and their correspondingdipole moments are: hexane (with a dipole moment of zero (0)), water(with a dipole moment of 1.85), and dimethylformamide (DMF) (with adipole moment of 3.82).

Hydrophilic Fluid

The hydrophilic fluid of the present invention includes a hydrophilicliquid that are miscible with water, including water, aliphaticalcohols, alkylene glycols, di(alkylene) glycols, monoalkyl ethers ofalkylene glycols or di(alkylene) glycols, and various mixtures thereof.Suitable aliphatic alcohols contain no greater than 6 carbons and nogreater than 4 hydroxyls, such as methanol, ethanol, isopropanol, andglycerol. Suitable alkylene glycols contain no greater than 5 carbons,such as ethylene glycol, propylene glycol, and 1,2-butylene glycol.Particularly, the hydrophilic thermal transfer fluid comprises ethyleneglycol, propylene glycol, and mixtures thereof. Ethylene glycol andpropylene glycol are excellent antifreeze agents and also markedlyreduce the freezing point of water. Suitable di(alkylene) glycolscontain no greater than 10 carbons, such as diethylene glycol,triethylene glycol, tetraethylene glycol, and dipropylene glycol.Commercial antifreeze coolants often contain more than one glycolcompounds. For example, Prestone antifreeze coolant contains 95 to 100%of ethylene glycol and no greater than 5% of diethylene glycol. Themixture as used herein refers to a combination of two or morehydrophilic liquids. As used herein, the term “alkylene glycol” refersto a molecule having glycol functional moiety in its structure ingeneral, including alkylene glycol, alkylene glycols, di(alkylene)glycols, tri(alkylene) glycols, tetra(alkylene) glycols, and theirvarious derivatives, such as ethers and carboxylic esters.

The hydrophilic fluid may contain one or more hydrophilic molecules. Forexample, the hydrophilic thermal transfer fluid may contain water,aliphatic alcohols, alkylene glycols, or various mixtures thereof. Thehydrophilic thermal transfer fluid may be a two-component mixture whichcontains water and ethylene glycol in various proportions. Thehydrophilic thermal transfer fluid may contain about 0.1 to about 99.9%,about 20 to about 80%, about 40 to about 60%, or about 50% by volume ofwater.

Hydrophobic Fluid

The hydrophobic fluid used in the present invention may be selected froma wide variety of well-known organic oils (also known as base oils),including petroleum distillates, synthetic petroleum oils, greases,gels, oil-soluble polymer composition, vegetable oils, and combinationsthereof. Petroleum distillates, also known as mineral oils, generallyinclude paraffins, naphthenes and aromatics.

Synthetic petroleum oils are the major class of lubricants widely usedin various industries. Some examples include alkylaryls such asdodecylbenzenes, tetradecylbenzenes, dinonylbenzenes, anddi-(2-ethylhexyl)benzenes; polyphenyls such as biphenyls, terphenyls,and alkylated polyphenyls; fluorocarbons such aspolychlorotrifluoroethylenes and copolymers of perfluoroethylene andperfluoropropylene; polymerized olefins such as polybutylenes,polypropylenes, propylene-isobutylene copolymers, chlorinatedpolybutylenes, poly(1-octenes), and poly(1-decenes); organic phosphatessuch as triaryl or trialkyl phosphates, tricresyl phosphate, trioctylphosphate, and diethyl ester of decylphosphonic acid; and silicates suchas tetra(2-ethylhexyl)silicate, tetra(2-ethylbutyl)silicate, andhexa(2-ethylbutoxy)disiloxane. Other examples include polyol esters,polyglycols, polyphenyl ethers, polymeric tetrahydrofurans, andsilicones.

In one embodiment of the present invention, the hydrophobic fluid is adiester which is formed through the condensation of a dicarboxylic acid,such as adipic acid, azelaic acid, fumaric acid, maleic acid, phtalicacid, sebacic acid, suberic acid, and succinic acid, with a variety ofalcohols with both straight, cyclic, and branched chains, such as butylalcohol, dodecyl alcohol, ethylene glycol diethylene glycol monoether,2-ethylhexyl alcohol, isodecyl alcohol, hexyl alcohol, pentaerytheritol,propylene glycol, tridecyl alcohol, and trimethylolpropane. Modifieddicarboxylic acids, such as alkenyl malonic acids, alkyl succinic acids,and alkenyl succinic acids, can also be used. Specific examples of theseesters include dibutyl adipate, diisodecyl azelate, diisooctyl azelate,di-hexyl fumarate, dioctyl phthalate, didecyl phthalate,di(2-ethylhexyl)sebacate, dioctyl sebacate, dicicosyl sebacate, and the2-ethylhexyl diester of linoleic acid dimer, the complex ester formed byreacting one mole of sebacic acid with two moles of tetraethylene glycoland two moles of 2-ethylhexanoic acid.

In another embodiment, the hydrophobic fluid is a polyalphaolefin whichis formed through oligomerization of 1-olefines containing 2 to 32carbon atoms, or mixtures of such olefins. Some common alphaolefins are1-octene, 1-decene, and 1-dodecene. Examples of polyalphaolefins includepoly-1-octene, poly-1-decene, poly-1-dodecene, mixtures thereof, andmixed olefin-derived polyolefins. Polyalphaolefins are commerciallyavailable from various sources, including DURASYN® 162, 164, 166, 168,and 174 (BP-Amoco Chemicals, Naperville, Ill.), which have viscositiesof 6, 18, 32, 45, and 460 centistokes, respectively.

In yet another embodiment, the hydrophobic fluid is a polyol ester whichis formed through the condensation of a monocarboxylic acid containing 5to 12 carbons and a polyol and a polyol ether such as neopentyl glycol,trimethylolpropane, pentaerythritol, dipentaerythritol, andtripentaerythritol. Examples of commercially available polyol esters areROYCO® 500, ROYCO® 555, and ROYCO® 808. ROYCO® 500 contains about 95% ofpentaerythritol esters of saturated straight fatty acids with 5 to 10carbons, about 2% of tricresyl phosphate, about 2% ofN-phenyl-alpha-naphthylamine, and about 1% of other minor additives.ROYCO® 808 are about 30 to 40% by weight of trimethylolpropane esters ofheptanoic, caprylic and capric acids, 20 to 40% by weight oftrimethylolpropane esters of valeric and heptanoic acids, about 30 to40% by weight of neopentyl glycol esters of fatty acids, and other minoradditives. Generally, polyol esters have good oxidation and hydrolyticstability. The polyol ester for use herein preferably has a pour pointof about −100° C. or lower to −40° C. and a viscosity of about 2 to 100centistoke at 100° C.

In yet another embodiment, the hydrophobic fluid is a polyglycol whichis an alkylene oxide polymer or copolymer. The terminal hydroxyl groupsof a polyglycol can be further modified by esterification oretherification to generate another class of known synthetic oils.Interestingly, mixtures of propylene and ethylene oxides in thepolymerization process will produce a water soluble lubricant oil.Liquid or oil type polyglycols have lower viscosities and molecularweights of about 400, whereas 3,000 molecular weight polyglycols areviscous polymers at room temperature.

In yet another embodiment, the hydrophobic fluid is a combination of twoor more selected from the group consisting of petroleum distillates,synthetic petroleum oils, greases, gels, oil-soluble polymercomposition, and vegetable oils. Suitable examples include, but notlimited to, a mixture of two polyalphaolefins, a mixture of two polyolesters, a mixture of one polyalphaolefine and one polyol ester, amixture of three polyalphaolefins, a mixture of two polyalphaolefins andone polyol ester, a mixture of one polyalphaolefin and two polyolesters, and a mixture of three polyol esters. In all the embodiments,the thermal transfer fluid preferably has a viscosity of from about 1 toabout 1,000 centistokes, more preferably from about 2 to about 800centistokes, and most preferably from about 5 to about 500 centistokes.

In yet another embodiment, the hydrophobic fluid is grease which is madeby combining a petroleum or synthetic lubricating fluid with athickening agent. The thickeners are generally silica gel and fatty acidsoaps of lithium, calcium, strontium, sodium, aluminum, and barium. Thegrease formulation may also include coated clays, such as bentonite andhectorite clays coated with quaternary ammonium compounds. Sometimescarbon black is added as a thickener to enhance high-temperatureproperties of petroleum and synthetic lubricant greases. The addition oforganic pigments and powders which include arylurea compoundsindanthrene, ureides, and phthalocyanines provide high temperaturestability. Sometimes, solid powders such as graphite, molybdenumdisulfide, asbestos, talc, and zinc oxide are also added to provideboundary lubrication. Formulating the foregoing synthetic lubricant oilswith thickeners provides specialty greases. The synthetic lubricant oilsinclude, without limitation, diesters, polyalphaolefins, polyol esters,polyglycols, silicone-diester, and silicone lubricants. Nonmeltingthickeners are especially preferred such as copper phthalocyanine,arylureas, indanthrene, and organic surfactant coated clays.

Surfactant:

A variety of surfactants can be used in the present invention as adispersant to facilitate uniform dispersion of nanoparticles and toenhance stabilization of such dispersion as well. Typically, thesurfactants used in the present invention contain an lipophilichydrocarbon group and a polar functional hydrophilic group. The polarfunctional group can be of the class of carboxylate, ester, amine,amide, imide, hydroxyl, ether, nitrile, phosphate, sulfate, orsulfonate. The surfactant can be anionic, cationic, zwitterionic,amphoteric and ampholytic.

In one embodiment, the surfactant is anionic, including sulfonates suchas alkyl sulfonates, alkylbenzene sulfonates, alpha olefin sulfonates,paraffin sulfonates, and alkyl ester sulfonates; sulfates such as alkylsulfates, alkyl alkoxy sulfates, and alkyl alkoxylated sulfates;phosphates such as monoalkyl phosphates and dialkyl phosphates;phosphonates; carboxylates such as fatty acids, alkyl alkoxycarboxylates, sarcosinates, isethionates, and taurates. Specificexamples of carboxylates are sodium cocoyl isethionate, sodium methyloleoyl taurate, sodium laureth carboxylate, sodium tridecethcarboxylate, sodium lauryl sarcosinate, lauroyl sarcosine, and cocoylsarcosinate. Specific examples of sulfates include sodium dodecylsulfate, sodium lauryl sulfate, sodium laureth sulfate, sodium tridecethsulfate, sodium tridecyl sulfate, sodium cocyl sulfate, and lauricmonoglyceride sodium sulfate.

Suitable sulfonate surfactants include alkyl sulfonates, arylsulfonates, monoalkyl and dialkyl sulfosuccinates, and monoalkyl anddialkyl sulfosuccinamates. Each alkyl group independently contains abouttwo to twenty carbons and can also be ethoxylated with up to about 8units, preferably up to about 6 units, on average, e.g., 2, 3, or 4units, of ethylene oxide, per each alkyl group. Illustrative examples ofalky and aryl sulfonates are sodium tridecyl benzene sulfonate andsodium dodecylbenzene sulfonate.

Illustrative examples of sulfosuccinates include, but not limited to,dimethicone copolyol sulfosuccinate, diamyl sulfosuccinate, dicaprylsulfosuccinate, dicyclohexyl sulfosuccinate, diheptyl sulfosuccinate,dihexyl sulfosuccinate, diisobutyl sulfosuccinate, dioctylsulfosuccinate, C12-15 pareth sulfosuccinate, cetearyl sulfosuccinate,cocopolyglucose sulfosuccinate, cocoyl butyl gluceth-10 sulfosuccinate,deceth-5 sulfosuccinate, deceth-6 sulfosuccinate, dihydroxyethylsulfosuccinylundecylenate, hydrogenated cottonseed glyceridesulfosuccinate, isodecyl sulfosuccinate, isostearyl sulfosuccinate,laneth-5 sulfosuccinate, laureth sulfosuccinate, laureth-12sulfosuccinate, laureth-6 sulfosuccinate, laureth-9 sulfosuccinate,lauryl sulfosuccinate, nonoxynol-10 sulfosuccinate, oleth-3sulfosuccinate, oleyl sulfosuccinate, PEG-10 laurylcitratesulfosuccinate, sitosereth-14 sulfosuccinate, stearyl sulfosuccinate,tallow, tridecyl sulfosuccinate, ditridecyl sulfosuccinate, bisglycolricinosulfosuccinate, di(1,3-di-methylbutyl)sulfosuccinate, and siliconecopolyol sulfosuccinates. The structures of silicone copolyolsulfosuccinates are set forth in U.S. Pat. Nos. 4,717,498 and 4,849,127,which are both hereby incorporated by reference.

Illustrative examples of sulfosuccinamates include, but not limited to,lauramido-MEA sulfosuccinate, oleamido PEG-2 sulfosuccinate, cocamidoMIPA-sulfosuccinate, cocamido PEG-3 sulfosuccinate, isostearamidoMEA-sulfosuccinate, isostearamido MIPA-sulfosuccinate, lauramidoMEA-sulfosuccinate, lauramido PEG-2 sulfosuccinate, lauramido PEG-5sulfosuccinate, myristamido MEA-sulfosuccinate, oleamidoMEA-sulfosuccinate, oleamido PIPA-sulfosuccinate, oleamido PEG-2sulfosuccinate, palmitamido PEG-2 sulfosuccinate, palmitoleamido PEG-2sulfosuccinate, PEG-4 cocamido MIPA-sulfosuccinate, ricinoleamidoMEA-sulfosuccinate, stearamido MEA-sulfosuccinate, stearylsulfosuccinamate, tallamido MEA-sulfosuccinate, tallow sulfosuccinamate,tallowamido MEA-sulfosuccinate, undecylenamido MEA-sulfosuccinate,undecylenamido PEG-2 sulfosuccinate, wheat germamido MEA-sulfosuccinate,and wheat germamido PEG-2 sulfosuccinate.

Some examples of commercial sulfonates are AEROSOL® OT-S, AEROSOL®OT-MSO, AEROSOL® TR70% (Cytec inc, West Paterson, N.J.), NaSul CA-HT3(King industries, Norwalk, Conn.), and C500 (Crompton Co, West Hill,Ontario, Canada). AEROSOL® OT-S is sodium dioctyl sulfosuccinate inpetroleum distillate. AEROSOL® OT-MSO also contains sodium dioctylsulfosuccinate. AEROSOL® TR70% is sodium bistridecyl sulfosuccinate inmixture of ethanol and water. NaSul CA-HT3 is calcium dinonylnaphthalenesulfonate/carboxylate complex. C500 is an oil soluble calcium sulfonate.

For an anionic surfactant, the counter ion is typically sodium but mayalternatively be potassium, lithium, calcium, magnesium, ammonium,amines (primary, secondary, tertiary or quandary) or other organicbases. Exemplary amines include isopropylamine, ethanolamine,diethanolamine, and triethanolamine. Mixtures of the above cations mayalso be used.

In another embodiment, the surfactant is cationic, including primarilyorganic amines, primary, secondary, tertiary or quaternary. For acationic surfactant, the counter ion can be chloride, bromide,methosulfate, ethosulfate, lactate, saccharinate, acetate and phosphate.Examples of cationic amines include polyethoxylated oleyl/stearyl amine,ethoxylated tallow amine, cocoalkylamine, oleylamine, and tallow alkylamine.

Examples of quaternary amines with a single long alkyl group are cetyltrimethyl ammonium bromide (“CTAB”), dodecyltrimethylammonium bromide,myristyl trimethyl ammonium bromide, stearyl dimethyl benzyl ammoniumchloride, oleyl dimethyl benzyl ammonium chloride, lauryl trimethylammonium methosulfate (also known as cocotrimonium methosulfate),cetyl-dimethyl hydroxyethyl ammonium dihydrogen phosphate,bassuamidopropylkonium chloride, cocotrimonium chloride,distearyldimonium chloride, wheat germ-amidopropalkonium chloride,stearyl octyidimonium methosulfate, isostearaminopropal-konium chloride,dihydroxypropyl PEG-5 linoleammonium chloride, PEG-2 stearmoniumchloride, behentrimonium chloride, dicetyl dimonium chloride, tallowtrimonium chloride and behenamidopropyl ethyl dimonium ethosulfate.

Examples of quaternary amines with two long alkyl groups aredistearyldimonium chloride, dicetyl dimonium chloride, stearyloctyldimonium methosulfate, dihydrogenated palmoylethylhydroxyethylmonium methosulfate, dipalmitoylethyl hydroxyethylmoniummethosulfate, dioleoylethyl hydroxyethylmonium methosulfate, andhydroxypropyl bisstearyldimonium chloride.

Quaternary ammonium compounds of imidazoline derivatives include, forexample, isostearyl benzylimidonium chloride, cocoyl benzyl hydroxyethylimidazoliniurn chloride, cocoyl hydroxyethylimidazolinium PG-chloridephosphate, and stearyl hydroxyethylimidonium chloride. Otherheterocyclic quaternary ammonium compounds, such as dodecylpyridiniumchloride, can also be used.

In yet another embodiment, the surfactant is zwitterionic, which hasboth a formal positive and negative charge on the same molecule. Thepositive charge group can be quaternary ammonium, phosphonium, orsulfonium, whereas the negative charge group can be carboxylate,sulfonate, sulfate, phosphate or phosphonate. Similar to other classesof surfactants, the hydrophobic moiety may contain one or more long,straight, cyclic, or branched, aliphatic chains of about 8 to 18 carbonatoms. Specific examples of zwitterionic surfactants include alkylbetaines such as cocodimethyl carboxymethyl betaine, lauryl dimethylcarboxymethyl betaine, lauryl dimethyl alpha-carboxyethyl betaine, cetyldimethyl carboxymethyl betaine, lauryl bis-(2-hydroxyethyl)carboxymethyl betaine, stearyl bis-(2-hydroxypropyl)carboxymethyl betaine,oleyl dimethyl gamma-carboxypropyl betaine, and laurylbis-(2-hydroxypropyl)alphacarboxy-ethyl betaine, amidopropyl betaines;and alkyl sultaines such as cocodimethyl sulfopropyl betaine,stearyidimethyl sulfopropyl betaine, lauryl dimethyl sulfoethyl betaine,lauryl bis-(2-hydroxyethyl)sulfopropyl betaine, andalkylamidopropylhydroxy sultaines.

In yet another embodiment, the surfactant is amphoteric. Suitableexamples of suitable amphoteric surfactants include ammonium orsubstituted ammonium salts of alkyl amphocarboxy glycinates and alkylamphocarboxypropionates, alkyl amphodipropionates, alkylamphodiacetates, alkyl amphoglycinates, and alkyl amphopropionates, aswell as alkyl iminopropionates, alkyl iminodipropionates, and alkylamphopropylsulfonates. Specific examples are cocoamphoacetate,cocoamphopropionate, cocoamphodiacetate, lauroamphoacetate,lauroamphodiacetate, lauroamphodipropionate, lauroamphodiacetate,cocoamphopropyl sulfonate, caproamphodiacetate, caproamphoacetate,caproamphodipropionate, and stearoamphoacetate.

In yet another embodiment, the surfactant is a polymer such asN-substituted polyisobutenyl succinimides and succinates, alkylmethacrylate vinyl pyrrolidinone copolymers, alkylmethacrylate-dialkylaminoethyl methacrylate copolymers,alkylmethacrylate polyethylene glycol methacrylate copolymers, andpolystearamides.

In yet another embodiment, the surfactant is an oil-based dispersant,which includes alkylsuccinimide, succinate esters, high molecular weightamines, and Mannich base and phosphoric acid derivatives. Some specificexamples are polyisobutenyl succinimide-polyethylenepolyamine,polyisobutenyl succinic ester, polyisobutenylhydroxybenzyl-polyethylenepolyamine, and bis-hydroxypropyl phosphorate.

In yet another embodiment, the surfactant used in the present inventionis a combination of two or more selected from the group consisting ofanionic, cationic, zwitterionic, amphoteric, and ampholytic surfactants.Suitable examples of a combination of two or more surfactants of thesame type include, but not limited to, a mixture of two anionicsurfactants, a mixture of three anionic surfactants, a mixture of fouranionic surfactants, a mixture of two cationic surfactants, a mixture ofthree cationic surfactants, a mixture of four cationic surfactants, amixture of two nonionic surfactants, a mixture of three nonionicsurfactants, a mixture of four nonionic surfactants, a mixture of twozwitterionic surfactants, a mixture of three zwitterionic surfactants, amixture of four zwitterionic surfactants, a mixture of two amphotericsurfactants, a mixture of three amphoteric surfactants, a mixture offour amphoteric surfactants, a mixture of two ampholytic surfactants, amixture of three ampholytic surfactants, and a mixture of fourampholytic surfactants.

Suitable examples of a combination of two surfactants of the differenttypes include, but not limited to, a mixture of one anionic and onecationic surfactant, a mixture of one anionic and one zwitterionicsurfactant, a mixture of one anionic and one amphoteric surfactant, amixture of one anionic and one ampholytic surfactant, a mixture of onecationic and one zwitterionic surfactant, a mixture of one cationic andone amphoteric surfactant, a mixture of one cationic and one ampholyticsurfactant, a mixture of one nonionic and one zwitterionic surfactant, amixture of one nonionic and one amphoteric surfactant, a mixture of onenonionic and one ampholytic surfactant, a mixture of one zwitterionicand one amphoteric surfactant, a mixture of one zwitterionic and oneampholytic surfactant, and a mixture of one amphoteric and oneampholytic surfactant. A combination of two or more surfactants of thesame type, e.g., a mixture of two anionic surfactants, is also includedin the present invention.

The Other Chemical Additives:

The nanofluids of the present invention may also contain one or moreother chemicals to provide other desired chemical and physicalproperties and characteristics, depending on whether they arehydrophobic or hydrophilic. In addition to the chemicals discussedseparately below for hydrophilic and hydrophobic polymer or thermaltransfer fluids, many other known types of additives such as dyes andair release agents, can also be included in finished compositionsproduced and/or used in the practice of the present invention. Ingeneral, the additive components are employed in nanofluids in minoramounts sufficient to enhance the performance characteristics andproperties of the base fluid. The amounts will thus vary in accordancewith such factors as the viscosity characteristics of the base fluidemployed, the viscosity characteristics desired in the finished fluid,the service conditions for which the finished fluid is intended, and theperformance characteristics desired in the finished fluid.

Suitable chemical additives for a fluid include, but are not limited to,buffering agents, corrosion inhibitors, defoamers, scale inhibitors, anddyes.

The buffering agents may be selected from any known or commonly usedbuffering agents. It will be appreciated by those skilled in the artthat selected buffering agents can exhibit both anti-corrosion andbuffering properties. In certain formulations, for example, benzoates,borates, and phosphates can provide both buffering and anti-corrosionadvantages. In addition, a base can be used to adjust the pH value of ananofluid. Illustrative examples of bases for use with this inventioninclude commonly known and used bases, for example, inorganic bases suchas KOH, NaOH, NaHCO₃, K₂CO₃, and Na₂CO₃. Therefore, the buffering systemand base can be adapted to provide a nanofluid composition with a pHlevel between 7.5 and about 11.

The corrosion inhibitors may be either an organic additive or aninorganic additive. Suitable organic anti-corrosive additives includeshort aliphatic dicarboxylic acids such as maleic acid, succinic acid,and adipic acid; triazoles such as benzotriazole and tolytriazole;thiazoles such as mercaptobenzothiazole; thiadiazoles such as2-mercapto-5-hydrocarbylthio-1,3,4-thiadiazoles,2-mercapto-5-hydrocarbyldithio-1,3,4-thiadiazoles,2,5-bis(hydrocarbylthio)-1,3,4-thiadiazoles, and2,5-(bis)hydrocarbyldithio)-1,3,4-thiadiazoles; sulfonates; andimidazolines. Suitable inorganic additives include borates, phosphates,silicates, nitrates, nitrites, and molybdates.

Suitable defoamers include components such as silicon defoamers,alcohols such as polyethoxylated glycol, polypropoxylated glycol oracetylenic glycols.

Suitable scale inhibitors include components such as phosphate esters,phosphino carboxylate, polyacrylates, polymethacylate, styrene-maleicanhydride, sulfonates, maleic anhydride co-polymer, andacrylate-sulfonate co-polymer.

The basic composition of the nanofluids of the present invention can betailored for selective applications. For example, nitrates and silicatesare known to provide aluminum protection. Borates and nitrites can beadded for ferrous metal protection, and benzotriazole and tolytriazolecan be added for copper and brass protection.

Suitable chemical additives for a hydrophobic fluid include, but are notlimited to, antioxidants, corrosion inhibitors, copper corrosioninhibitors, friction modifiers, viscosity improvers, pour pointdepressants, and seal-swelling agents.

Suitable antioxidants include phenolic antioxidants, aromatic amineantioxidants, sulfurized phenolic antioxidants, and organic phosphates.Examples include 2,6-di-tert-butylphenol, liquid mixtures of tertiarybutylated phenols, 2,6-di-tert-butyl-4-methylphenol,4,4′-methylenebis(2,6-di-tert-butylphenol),2,2′-methylenebis(4-methyl-6-tert-butylphenol), mixed methylene-bridgedpolyalkyl phenols, 4,4′-thiobis(2-methyl-6-tert-butylphenol),N,N′-di-sec-butyl-p-phenylenediamine, 4-isopropylaminodiphenylamine,phenyl-alpha-naphthylamine, and phenyl-beta-naphthylamine.

Suitable corrosion inhibitors include dimer and trimer acids, such asthose produced from tall oil fatty acids, oleic acid, or linoleic acid;alkenyl succinic acid and alkenyl succinic anhydride corrosioninhibitors, such as tetrapropenylsuccinic acid, tetrapropenylsuccinicanhydride, tetradecenylsuccinic acid, tetradecenylsuccinic anhydride,hexadecenylsuccinic acid, hexadecenylsuccinic anhydride; and the halfesters of alkenyl succinic acids having 8 to 24 carbon atoms in thealkenyl group with alcohols such as the polyglycols. Other suitablecorrosion inhibitors include ether amines; acid phosphates; amines;polyethoxylated compounds such as ethoxylated amines, ethoxylatedphenols, and ethoxylated alcohols; imidazolines; aminosuccinic acids orderivatives thereof.

Suitable copper corrosion inhibitors include thiazoles such as2-mercapto benzothiazole; triazoles such as benzotriazole,tolyltriazole, octyltriazole, decyltriazole, and dodecyltriazole; andthiadiazoles such as 2-mercapto-5-hydrocarbylthio-1,3,4-thiadiazoles,2-mercapto-5-hydrocarbyldithio-1,3,4-thiadiazoles,2,5-bis(hydrocarbylthio)-1,3,4-thiadiazoles, and2,5-(bis)hydrocarbyldithio)-1,3,4-thiadiazoles.

Suitable friction modifiers include aliphatic amines, aliphatic fattyacid amides, aliphatic carboxylic acids, aliphatic carboxylic esters,aliphatic carboxylic ester-amides, aliphatic phosphonates, aliphaticphosphates, aliphatic thiophosphonates, and aliphatic thiophosphates,wherein the aliphatic group usually contains above about eight carbonatoms so as to render the compound suitably oil soluble. Also suitableare aliphatic substituted succinimides formed by reacting one or morealiphatic succinic acids or anhydrides with ammonia.

Suitable viscosity improvers include olefin copolymers,polymethacrylates, hydrogenated styrene-diene, and styrene-polyesterpolymers. Also suitable are acrylic polymers such as polyacrylic acidand sodium polyacrylate; high-molecular-weight polymers of ethyleneoxide; cellulose compounds such as carboxymethylcellulose; polyvinylalcohol; polyvinyl pyrrolidone; xanthan gums and guar gums;polysaccharides; alkanolamides; amine salts of polyamide;hydrophobically modified ethylene oxide urethane; silicates; and fillerssuch as mica, silicas, cellulose, wood flour, clays (includingorganoclays) and nanoclays; and resin polymers such as polyvinyl butyralresins, polyurethane resins, acrylic resins and epoxy resins.

Most pour point depressants are organic polymers, although somenonpolymeric substances have been shown to be effective. Bothnonpolymeric and polymeric depressants can be used in the presentinvention. Examples include alkylnaphthalenes, polymethacrylates,polyfumarates, styrene esters, oligomerized alkylphenols, phthalic acidesters, ethylenevinyl acetate copolymers, and other mixed hydrocarbonpolymers. The treatment level of these additives is usually low. Innearly all cases, there is an optimum concentration above and belowwhich pour point depressants become less effective.

Suitable seal-swelling agents include dialkyl diesters of adipic,azelaic, sebacic, and phthalic acids. Examples of such materials includen-octyl, 2-ethylhexyl, isodecyl, and tridecyl diesters of adipic acid,azelaic acid, and sebacic acid, and n-butyl, isobutyl, pentyl, hexyl,heptyl, octyl, nonyl, decyl, undecyl, dodecyl, and tridecyl diesters ofphthalic acid. Also useful are aromatic hydrocarbons with suitableviscosity.

In addition to the chemicals listed, many other known types of additivessuch as dyes, foam inhibitors, demulsifiers, and air release agents, canalso be included in finished compositions produced and/or used in thepractice of the present invention. In general, the additive componentsare employed in nanofluids in minor amounts sufficient to enhance theperformance characteristics and properties of the base fluid. Theamounts will thus vary in accordance with such factors as the viscositycharacteristics of the base fluid employed, the viscositycharacteristics desired in the finished fluid, the service conditionsfor which the finished fluid is intended, and the performancecharacteristics desired in the finished fluid.

Physical Agitation:

The nanofluid of the present invention may be prepared by anyconventional means of dispersing a mixture of the appropriate carbonnanoparticles, metal oxide nanoparticles, surfactant(s), and/or otheroptional chemical additives. For example, a common approach is using aphysical method to form a stable suspension of nanoparticles in a fluid.A variety of physical mixing methods are suitable for use in the presentinvention, including a conventional mortar and pestle mixing, high shearmixing, such as with a high speed mixer, homogenizers, microfluidizers,high impact mixing, and ultrasonication methods.

Among these methods, ultrasonication is one of the least destructive tothe structures of carbon nanoparticles. Ultrasonication can be doneeither in the bath-type ultrasonicator, or by the tip-typeultrasonicator. Typically, tip-type ultrasonication is for applicationswhich require higher energy output. Ultrasonication at a intermediateintensity for up to 60 minutes (min), and usually in a range of from 10to 30 minutes is desired to achieve better homogeneity. Additionally,the mixture is ultrasonicated intermittently to avoid overheating. It iswell known that overheating can cause covalent bond breakage in a carbonnanotube, which cause the nanotube to lost its beneficial physicalproperties. As such, the carbon nanoparticle-containing mixture isgenerally energized for a predetermined period of time with a break inbetween. Each energizing period is no more than about 30 min, no morethan about 15 min, no more than 10 min, no more than 5 min, no more than2 min, no more than 1 min, or no more than 30 seconds. The break betweenultrasonication pulses provides the opportunity for the energized carbonnanoparticles to dissipate the energy. The break is typically no lessthan about 1 min, no less than about 2 min, no less than about 5 min, orbetween about 5 to about 10 min.

The raw material mixture may also be pulverized by any suitable knowndry or wet grinding method. One grinding method includes pulverizing theraw material mixture in the fluid mixture of the present invention toobtain a concentrate, and the pulverized product may then be dispersedfurther in a liquid medium with the aid of the dispersants describedabove. However, pulverization or milling often reduces the carbonnanotube average aspect ratio.

It will be appreciated that the individual components can be separatelyblended into the thermal transfer fluid, or can be blended therein invarious subcombinations, if desired. Ordinarily, the particular sequenceof such blending steps is not critical. Moreover, such components can beblended in the form of separate solutions in a diluent. It ispreferable, however, to blend the components used in the form of anadditive concentrate, as this simplifies the blending operations,reduces the likelihood of blending errors, and takes advantage of thecompatibility and solubility characteristics afforded by the overallconcentrate.

Physical agitation methods particularly suitable for making nanogreaseare those employing relatively high shearing or dispersing devices,including, but not limited to, Morehouse mills, Buxton knife mills,Gaulin homogenizers, colloid mills, rotating knife-edge mills,rotor-stator mills, and three-roll mills. In an exemplary embodiment,after a final grease composition is achieved, the resulting grease isgenerally passed one or more times through one of these shearing ordispersing devices to enhance the characteristics (e.g., smoothness,shear stability, oil separation and bleed properties) and to maximizethe thickening power of a grease thickener, such as carbon nanotubes.

Formulation:

The nanofluid of the present invention is a dispersion ofnonmagnetically sensitive nanoparticles, magnetically sensitivenanoparticles, and surfactant(s) in a fluid. The nonmagneticallysensitive nanoparticles, magnetically sensitive nanoparticles, andsurfactant(s) may be dispersed in the fluid separately or as ananoparticle mixture or suspension. The nanoparticle mixture orsuspension may include nonmagnetically sensitive particles, magneticallysensitive particles, and surfactant(s).

In one embodiment, the nanofluid contains no less than about 80%, noless than about 85%, no less than about 90%, or no less than about 95%by weight of a fluid. The nanofluid contains no greater than about 30%,no greater than about 15%, no greater than about 10%, no greater thanabout 5%, no greater than about 2.5%, or no greater than about 1%, nogreater than about 0.5%, no greater than about 0.2%, no greater thanabout 0.1%, or no greater than about 0.05% by weight of nanoparticles.

In one embodiment, the magnetically sensitive nanoparticles aremagnetically sensitive metals and metal oxides, and more preferably,iron oxide (Fe₂O₃). In another embodiment, the nonmagnetically sensitivenanoparticles are carbon particles, carbon nanotubes.

In another aspect, the nanofluids of the present invention havecombinations of specific pH ranges and surfactant(s). In one embodiment,if the surfactant(s) have a net negative charge the pH of the fluid isgreater than about 5. In another embodiment, if the surfactant(s) have anet positive charge, the pH of the fluid is less than about 10.

The nanoparticles of the present invention are attached to each other incombinations of two or more nanoparticles. In one embodiment,magnetically sensitive nanoparticles are attached to nonmagneticallysensitive particles. In a preferred embodiment, the nanofluid containsno less than about no less than about 90%, no less than 95%, or no lessthan 98% by weight of attached nanoparticles. The nanoparticles may beattached to each other through any conventional methods including:chemical bonding and Ni coating. However, a preferred method ofattachment is electrostatic attraction. As opposed to chemical and otherbonding means, through electrostatic attraction, the conjugatedstructure (in this instance, for example, the S/NSP Complex bound to amagnetically sensitive nanoparticle) is maintained, and not altered.

The nanofluids of the present invention include one or more surfactants.The nanofluid contains from about no greater than 10%, no greater than1%, no greater than 0.5%, no greater than 0.2%, from 0.1 to about 30%,from about 1 to about 20%, from about 1 to about 15%, or from about 1 toabout 10% by weight of surfactant(s).

The nanofluid may further comprise other additives to improve chemicaland/or physical properties. Typically, the amount of these additivestogether is no greater than about 10% by weight of the nanofluid.Nevertheless, the total amount of all the ingredients of the nanofluidtogether should equal to 100%.

Upon exposure to a magnetic field, the nanofluids of the presentinvention exhibit enhanced polymer characteristics and thermalconductivities. For example, upon exposure to a magnetic field, thenanofluids of the present invention exhibit an increase of thermalconductivity of at least about 50%, at least about 60%, at least about70%, at least about 80%, or at least about 90%. The thermal conductivityenhancement depends upon various factors, including the components inthe nanofluid, the time of exposure to the magnetic field, and theamount of attachment of the components and/or detachment of the attachedcomponents in the nanofluid.

The nanofluids may be prepared by dispersing the nanoparticles, alongwith the surfactant(s), directly into a mixture of a fluid separately,or as part of a pre-mixture or suspension, and other additives with aphysical agitation, such as ultrasonication. However, the order ofaddition of the individual or attached components is not critical forthe practice of the invention.

Magnetic Field:

Magnets and magnet field generating devices are well-known. The magneticfield used in the present invention may be generated by any conventionalmeans for creating a magnetic field. Common magnets or devices thatgenerate a magnetic field include, but are not limited to: permanentmagnets, magnetic materials which create a changing magnetic field,ferromagnetic components, and solenoid magnets. The magnets or magneticmaterial may be fluxing, fixed, moving or otherwise, and may createpulsed, changing, fluxing, modulating, and/or fixed/constant magnetic,waved or energy fields (as a few examples). They may includingalternating poles, north poles, south poles, or combinations thereof,and different shapes of the magnets and magnetic fields, all within thesame magnet or magnetic layer.

The magnetic fields may be of any strength, which is typically measuredas Gauss or Tesla units (with one (1) Tesla=10⁴ Gauss). Generally, thespeed of alignment increases with increasing magnetic field strength.Therefore, magnetic field generators or magnets with various strengthsmay be selected to provide a desirable alignment speed.

A simple way to create a magnetic field of the present invention is toprovide by a pair of spaced, Ba-ferrite magnet plates. A magnetic fieldis created in-between the gap of the two plates. To be exposed to themagnetic field generated by the magnet plates, the nanofluid is placedin the gap in-between the magnetic plates.

Magnetic particles in a liquid medium can assume a variety ofconfigurations, depending on the nature of the magnetic particles andthe strength of the magnetic field (McCormack, et al., J. Electron.Mater. 23 (1994) 715-720; Philip, et al., Appl. Phys. Lett. 92 (2008)043108-043110; Shima, et al., J. Phys. Chem. 114 (2010) 18825-18833;Zhu, et al., Appl. Phys. Lett. 89 (2006) 023123-023125). Under amagnetic field, small magnetic particles form interconnected networksand tend to get become spatially oriented along the magnetic field. Thisin turn, moves the carbon nanotubes nearby and induces more physicalcontacts, which is anticipated to improve thermal conductivity (Wright,et al., Appl. Phys. Lett. 91 (2007) 173116-173118; Horton, et al., J.Appl. Phys. 107 (2010) 104320-104322).

The amount of alignment and therefore, amount of enhanced polymer andthermal conductivity, is related to the amount of time of exposure to amagnetic field. It was found that at a certain period of time ofexposure to a magnetic field (e.g., Tmax), alignment reaches a maximumand the enhanced or increased polymer characteristics or thermalconductivity will also reach a maximum. After this Tmax, there was foundto be either no further increased alignment (and enhanced or increasedpolymer characteristics or thermal conductivity) or reduced alignment(and reduced or decreased polymer characteristics or thermalconductivity) from Tmax. For example, thermal conductivity was measuredwith 0.017 wt % SWNT, 0.017 wt % Fe₂O₃ and 0.17 wt % NaDDBS in DI waterunder a magnetic field of 0.62 kG. Thermal conductivity was measured atthe following time intervals (with the corresponding thermalconductivity (TC) values: 5 seconds (0.53), 10 seconds (0.93), 30seconds (1.23) and 60 seconds (0.70).

EXPERIMENTS

Single wall carbon nanotubes (SWNT) were purchased from Helix MaterialSolutions Inc. in Richardson, Tex. The magnetically sensitive Fe2O3 nanoparticles with an average diameter of 5-25 nm, and chemical surfactantsodium dodecylbenzene sulfonate (NaDDBS) were purchased from SigmaAldrich.

Sonication was performed using a Branson Digital Sonifier, model 450. Amagnetic field was provided by a pair of spaced, Ba-ferrite magnetplates (4×6×1 inch). The sample was placed in the middle of gap betweenthe magnets. In order to magnetize the Fe2O3 particles, samples wereexposed to a magnetic field for approximately one hour before usage.

A Redlake Model PCI 2000S Motion Scope (MASD Inc, San Diego, Calif.) wasused to observe the behavior of nano particles mixture. The detectionparameters used were: record rate 250, Shutter 1/250, trigger 70%. Thelens were a WHB 10×/20 and MPlan 10×/0.25. Images of the solvent effectswere observed using an Olympus IX71 optical microscope and recordedusing a Princeton Instruments PIXIS CCD camera.

The pH values were measured using a pH Mettler Toledo model SevenEAsyS20. The thermal conductivity data was obtained using a Hot Disk™thermal constants analyzer (whose product details may be found athttp://www.hotdisk.se, which was last accessed on Sep. 14, 2011), usingthe following parameters: measurement depth of 6 mm, room temperature,power of 0.012 W, measurement time of 15 s, sensor radius of 3.189 mm,temperature coefficient of resistance of 0.0471/K, disk type Kapton, andtemperature drift rec yes. The uncertainty of the thermal conductivitiesin the nanofluids was within 3%.

The magnetic field intensity was recorded using a F.W. Bell GaussmeterModel 5060. Scanning electron microscopy (SEM) images were acquiredusing the backscattered electron detector on a Zeiss Supra40VP variablepressure system.

Experiment 1 Microscopy Showing Nanoparticle Alignment

Microscope images of 0.017 wt % SWNT, 0.017 wt % Fe2O3 and 0.17 wt %NaDDBS in DI water were obtained using the high speed microscope videosystem—A: 0 min; B: 0.5 min; C: 1 min; D: 2 min are shown in FIG. 1. Amagnetic field (H=0.62 kG) was applied with an internal reference of 30μm. As shown in FIG. 1A, it is clearly apparent that at zero min thecarbon nanotube, metal oxide Fe2O3 and surfactant NaDDBS mixtures arerandomly dispersed in the water. As is the case for pristine nanotubes,these mixtures are also entangled and look like scattered dots (most ofthese dots are much less than 30 μm in diameter) in the microscopeimage. With the addition of the external magnetic field, the “scattereddots” start to stretch, vibrate and align, as shown in FIG. 1B, asquickly as 30 seconds after the application of the magnetic field. Astime progresses, FIG. 1C—approximately 1 min after the application ofthe magnetic field, the aligned nanoparticles continue to vibrate andenlarge. Eventually, at approximately 2 minutes (FIG. 1D), theserandomly dispersed dots form larger and longer lines, indicatingaggregated and entangled nanotubes and metal oxide mixtures that havebeen formed to create aligned chains and clusters under the externalmagnetic field.

FIG. 2 illustrates the microscopic images of a 0.017 wt % SWNT, 0.017 wt% Fe2O3 and 0.17 wt % NaDDBS solution in DI water as observed using thedigital camera. FIG. 2A illustrates the image before the magnetic field,while FIG. 2B illustrates the effect of the magnetic field (H=0.62 kG).Internal reference is 30 μm. The images confirm what we discussed inFIG. 1 and give the scene in large area. The aligned chains in theimages appear to be continuous, indicating that these mixtures arealigned, but form chains and clusters.

It is intuitive that the particles would move towards the direction ofmagnetic field. However, these nanotube and metal oxide mixtures do notexhibit significant movement under the influence of the magnetic field.Instead, the aligned nanotube lines continue to stretch longer and movecloser together, forming longer and thicker lines. After some time (>12h), they start to precipitate, and lines of black particles along themagnetic field at the bottom of vessel are clearly observed.

This observed phenomenon coincides very well with the previouslyreported time dependent thermal conductivity results of carbon nanotubesand metal oxides in water (Hong, et al., Synth. Met. 157 (2007)437-440). Without carbon nanotubes and the application of the magneticfield, the thermal conductivity value of Fe₂O₃ nanofluids isapproximately 0.62˜0.63 W/m-K and remains nearly constant with respectto time. Because of the small number of contacts between the Fe2O3nanoparticles, the thermal conductivity value is essentially the same asthe value for the DI water itself. With the addition of carbonnanotubes, the thermal conductivity increases to approximately 0.70W/m·K and is apparently independent of time. The reasonable explanationfor the thermal conductivity enhancement from 0.62-0.63 to 0.70 W/m·K isdue to the aggregation of the metal oxide particles on the surface ofthe nanotubes by electrostatic attraction and the formation of theaggregated chain along the nanotubes (Wenzel, et al., Appl. Phys. Lett.92 (2008) 023110-023112).

As shown above, high speed microscopy was utilized to allow real timevisualization of the movement of single walled carbon nanotubes (SWNT)with magnetically sensitive nanoparticles (Fe₂O₃) and a chemicalsurfactant (NaDSSB) in water. Initially, entangled SWNT, Fe₂O₃ andNaDSSB mixtures were randomly dispersed in the fluid. Upon extendedexposure to the magnetic field, the mixture slowly vibrated, thenanoparticles straightened and aligned with respect to the magneticfield. The aligned nanoparticle chains appeared to be continuous andunbroken, forming a combination of aligned particles and clusters.

Experiment 2 Effect of Time of Exposure of Magnetic Field on ThermalConductivity

In the presence of the magnetic field, the thermal conductivity of thenanofluids demonstrates a very interesting behavior. The thermalconductivity initially increases with time, but eventually reaches apeak value of 0.95 W/m·K after exposure to a magnetic field for betweentwo to four minutes, indicating the impact of the nanotube alignmentprocess. Microscopy videos illustrate the time dependent stretching andorientation process of the nanotubes. As the time of exposure to themagnetic field increases, the thermal conductivity decreases. This isthought to be due to the excessive agglomeration of the nanoparticles,creating larger particles that begin to precipitate or settle in thefluid. This last point was confirmed by microscopic examination. Inaddition, the video images provide an explanation for the impact oflonger residence times in the magnetic field, where the thermalconductivity value decreases to approximately 0.63˜0.64 W/m·K, evenlower than that of the fluid with nanotubes and the magnetic particlesFe₂O₃ (0.70 W/m·K). This is attributed to the precipitation andsedimentation of the CNTs together with magnetic particles over extendedperiods of time and no more particle aggregation exists.

As observed in microscopy images, the carbon nanotube lines stretchlonger, move closer together and form longer and thicker lines under theinfluence of an external magnetic field and as a result, enhance theeffective thermal conductivity. In order to determine the influence ofthe time in residence, nanofluids (0.017 wt % SWNT, 0.017 wt % Fe₂O₃ and0.17 wt % NaDDBS in DI water) were placed in a 0.62 kG magnetic fieldfor times of 5 s, 10 s, 30 s, and 60 s. The magnetic field was thenremoved and the thermal conductivity value recorded. Table 1 below liststhe maximum observed thermal conductivity values and the increasedratios for various exposure times, which is also shown in FIG. 3.

TABLE 1 Effect of exposure time on the thermal conductivity ofnanofluids Maximum Thermal Exposure Time Conductivity^(a) ThermalConductivity All the time 0.95 0.53  5 1.18 0.93 10 1.30 1.13 30 1.361.23 60 1.04 0.70 ^(a)Thermal conductivity was measured with 0.017 wt %SWNT, 0.017 wt % Fe₂O₃ and 0.17 wt % NaDDBS in DI water under a magneticfield of 0.62 kG.

As shown in Table 1, with longer exposure to the external magneticfield, the thermal conductivity maximum value increases. At an exposuretime of 30 s, the thermal conductivity reaches a maximum value. Furtherexposure to the magnetic field results in a decrease in the thermalconductivity. It is interesting to note that continued exposure to anexternal magnetic field leads to the lowest thermal conductivity. Thiscoincides with the microscopy images of the carbon nanotube movement.The results indicate that the gradual magnetic clumping or clusteringwas the cause of the thermal conductivity decrease.

Because of the semi-continuous nature of these nanosuspensions and theinherent viscosity of the fluid, some minutes are required for themixtures to respond to the applied magnetic field and align. Timedependent thermal conductivity experiments indicate that the alignmentprocess dominates the thermal conductivity enhancement, as opposed tomicro convection.

Experiment 3 Solvent Effect on Alignment

The macrogeometrical effect of magnetically aligned 0.017 wt % SWNT,0.017 wt % Fe2O3 and 0.17 wt % NaDDBS in DI water was evaluated usingdigital camera images as shown in FIG. 4. It is clearly apparent thatthe black particles form lines along the magnetic field on the bottom ofvessel. While the trend of this alignment is the same, solvents withhigher dipole moments demonstrate a more rapid alignment. FIG. 5illustrates the optical microscope images for 0.017 wt % SWNT, 0.017 wt% Fe2O3 and 0.17 wt % NaDDBS in different solvents: (a) Hexane, (b)Water, and (c) DMF. The scale shown is 100 μm. It is well known thatdipole moments for Hexane, water, and DMF are 0, 1.85, and 3.82,respectively. Therefore, the alignment trends are different.

Experiment 4 Effect of Alignment Process on Thermal Conductivity

In order to ensure that the alignment process is the dominant factorinfluencing the thermal conductivity enhancement (as opposed to microconvection), the position of the magnetic field was modified (and hencethe magnetic field intensity and orientation) during the time dependentthermal conductivity measurements. This resulted in a change ofstability in the fluid. However, no significant differences in thethermal conductivity were observed. The influence of the direction ofthe magnetic field was observed by manually switching the magnets duringthe experimental tests to determine if the tangling and contacts amongnanotubes, metal oxides and chemical surfactants would be affected.Again, the thermal conductivity did not show any significantdifferences. Further evidence indicates that the thermal conductivityenhancement could be observed along the applied magnetic fielddirection, but not along the perpendicular direction (data not shown).If the micro convection assumption is true, then thermal conductivityenhancement in all directions should be comparable. Normally, microconvection effects only last several minutes. The longer time scale ofthe thermal conductivity enhancement presents strong evidence thatthermal conductivity enhancement is not due to micro convection (Shima,et al., Appl. Phys. Lett. 95 (2009) 133112-133114).

The above microscope and thermal conductivity results demonstrate thatalignment and orientation of the nanotubes in a fluid are critical andessential to the enhancement of the thermal conductivity of thecomposite fluid. However, the increase in the ratio of the thermalconductivity is not as significant as anticipated due to the thermalcontact resistance in the nanofluids (Bahrami, et al., J. Heat Transfer126 (2004) 896-905).

Example 5 Effect of Electrostatic Attraction on Aggregation

To verify whether electrostatic attraction between nanotube/surfactantand metal oxides causes aggregation, the charge of the surfactant waschanged from a negative charge surfactant (e.g., NaSDDB) to a positivecharge surfactant (e.g., CTAB).

As illustrated in FIGS. 6A and 6B respectively, it is clearly apparentthat under a magnetic field, the nanotubes align very well in thedirection of the magnetic field, either in scale bar 100 μm (A) or 10 μm(B). To verify this, as part of the investigation, drops of the SEMsamples containing nanotube, Fe2O3, and NaDSSB were placed on the SEMsample holder and allowed to dry while influenced by the magnetic field.Because extended exposure to magnetic fields will enlarge the alignedchains and clusters and cause them to precipitate, the samples were onlyexposed to the magnetic field for a short time (2-3 mins). During thattime, the samples exhibited near perfect alignment, which resulted in asubstantially improved thermal conductivity.

Due to the technical limitations of the optical microscope, it is verydifficult to obtain real images of the aggregation among the individualcarbon nanotubes, the surfactant and the metal oxide. Based upon theinitial assumptions, the electrostatic attraction between the nanotube,surfactant and metal oxide causes the aggregation to occur. It isinteresting to speculate on the following: If the charge of thesurfactant is changed, for example from a negative charge (NaSDDB) to apositive charge (CTAB), then no electrostatic attraction would occur andthe Fe₂O₃ nanoparticles should separate from the nanotube colloidaldomain. To verify this hypothesis, microscopy videos of 0.017 wt % SWNT,0.017 wt % Fe₂O₃ and 0.17 wt % CTAB in DI water were recorded. FIG. 7shows the microscopic images extracted from the video at: A=0 min; B=0.5min; C=1 min; and D=2 mins. The magnetic field (H=0.62 kG) was appliedwith an internal reference of 30 μm.

It is clearly evident that initially (FIG. 7A), the carbon nanotube,metal oxide Fe2O3 and surfactant CTAB mixtures are randomly dispersed inthe water. These mixtures are also entangled and look like scattereddots in the microscope image. With the addition of the external magneticfield, the scattered dots start to vibrate, but do not yet stretch andalign, only slight and short lines are formed, as shown in FIG. 7 (0.5min after addition of magnetic field) and FIG. 7C (1 min after additionof magnetic field). At 2 mins. (FIG. 7D), the randomly dispersed blackdots (SWNT) remain relatively constant and do not yet appear to formlarger and longer lines, similar to FIG. 1. However, some fine lines(Fe₂O₃) are formed and alignment appears to begin to take place. Theselines are believed to be the Fe₂O₃ nanoparticles which supported by theimages provided. Although it is impossible to use Redlake CCD microscopeimage to tell the separation of nanoparticle from nanotube, But, by thetrend and macro-effect, we could see the images with electrostaticattraction and without electrostatic attraction are totally different.We also have performed the thermal conductivity measurements and foundthe results are totally different.

To further support this hypothesis, it would be beneficial to be able toobserve some real images of the Fe₂O₃ nanoparticle andnanotube/surfactants as they interact. Therefore, microscope images of0.051 wt % SWNT, 0.051 wt % Fe₂O₃ and 0.17 wt % CTAB in DI water wereextracted from the video at: A=0 min; B=2 mins. They are shown in FIG.8. A magnetic field (H=0.62 kG) was applied and the internal referenceis 30 μm. The high weight percentage (0.051 wt %) of SWNT and Fe₂O₃ andlow weight percentage (0.17 wt %) of CTAB exaggerates the dispersioncircumstance. As is clearly seen from FIG. 8A, some fine lines areentangled and separated as represented by the big black dots. It isassumed that the entangled fine lines are Fe₂O₃ nanoparticles and thelarge black dots are SWNT/surfactant. If this assumption is correct,then the Fe₂O₃ entangled lines will be aligned under the externalmagnetic field and the SWNT/surfactant will be inactive. FIG. 8Bverifies this assumption.

CONCLUSIONS

In summary, observations and real time images of the movement of SWNT,Fe₂O₃, and NaDSSB in water under a magnetic field using high speedmicroscopy were made. Initially, carbon nanotube, metal oxide Fe₂O₃, andsurfactant NaDSSB mixtures were randomly dispersed (entangled) in thefluid. Upon continued exposure to the magnetic field, the mixturegradually vibrated, stretched out and finally aligned. The alignedchains in the images were found to be endless. This indicates that thesemixtures are aligned, but form chains and clusters. Because of thesemi-continuous nature of these nano mixtures, as well as the viscosityresistance of the fluid itself, it takes some time for the mixture torespond to the applied magnetic field and to become aligned.

Time dependent thermal conductivity experimental results using differentmagnetic field intensities and orientations indicate that the alignmentprocess dominates the thermal conductivity enhancement rather than microconvection. Scanning Electron Microscopy (SEM) images also show that theSWNT and Fe₂O₃ are aligned well under the influence of a magnetic field.

The assumption that the electrostatic attraction between the nanotube,surfactant and metal oxide causes aggregation by changing the chargenature of surfactant from a negative charge (NaSDDB) to a positivecharge (CTAB) is verified.

The significance of electrostatic force induced alignment is thatperfect conjugated structure of carbon nanotube is maintained.Therefore, those nanotubes still show excellent thermal, electrical, andmechanical properties.

The examples set forth above are provided to give those of ordinaryskill in the art with a complete disclosure and description of how tomake and use the preferred embodiments of the compositions and themethods, and are not intended to limit the scope of what the inventorsregard as their invention. Modifications of the above-described modesfor carrying out the invention that are obvious to persons of skill inthe art are intended to be within the scope of the following claims. Allpublications, patents, and patent applications cited in thisspecification are incorporated herein by reference as if each suchpublication, patent or patent application were specifically andindividually indicated to be incorporated herein by reference.

We claim:
 1. A nanoparticle composition comprising: one or morenonmagnetically sensitive nanoparticles, one or more magneticallysensitive nanoparticles, and one or more surfactants; wherein the one ormore surfactants are selected from the group consisting of an ampholyticsurfactant, an amphoteric surfactant, an anionic surfactant, a cationicsurfactant, a zwitterionic surfactant, and combinations thereof; whereinthe one or more surfactants are attached to the one or morenonmagnetically sensitive nanoparticles forming a surfactant andnonmagnetically sensitive particle complex (S/NSP) by electrostaticattraction; wherein the S/NSP complex is attached to the one or moremagnetically sensitive nanoparticles by electrostatic attraction; andwherein at least a portion of the nonmagnetically sensitivenanoparticles are magnetically aligned.
 2. The nanoparticle compositionof claim 1 further comprising a fluid, wherein the fluid has a dipolemoment greater than about zero.
 3. The nanoparticle composition of claim2, wherein the fluid has a pH greater than about 5 and the surfactantshave a net negative charge, or the fluid has a pH less than about 9 andthe surfactants have a net positive charge.
 4. The nanoparticlecomposition of claim 1, wherein the magnetically sensitive particles aremagnetically sensitive metals, metal oxides, or combinations thereof. 5.The nanoparticle composition of claim 4, wherein the magneticallysensitive metal oxide is selected from the group consisting of Fe, Co,Fe₂O₃, and Fe₃O₄.
 6. The nanoparticle composition of claim 1, whereinthe magnetically sensitive particles comprise Fe₂O₃.
 7. The nanoparticlecomposition of claim 1, wherein the nonmagnetically sensitivenanoparticles are carbon nanotubes.
 8. The nanoparticle composition ofclaim 1, wherein the surfactants are selected from the group aconsisting of a betain, a carboxylate, a quaternary amine, a quaternaryammonium, a sulfate, a sulfonate, a sultaine, and combination thereof.9. The nanoparticle composition of claim 8, wherein the sulfonate issodium dodecylbenzene sulfonate, the quaternary ammonium is aheterocyclic quaternary ammonium, and the quaternary amine istrimethylammonium bromide.
 10. The nanoparticle composition of claim 1,wherein the one or more nonmagnetically sensitive nanoparticles compriseSWNT, DWNT, MWNT, graphite, graphene, modified carbon nanotubes, orfunctionalized carbon nanotubes; and wherein substantially all of thenonmagnetically sensitive nanoparticles are magnetically aligned.
 11. Ananofluid comprising: one or more nonmagnetically sensitivenanoparticles comprising carbon nanoparticles, one or more magneticallysensitive nanoparticles, and one or more surfactants; wherein thenonmagnetically sensitive nanoparticles, magnetically sensitivenanoparticles, and one or more surfactants are dispersed in a fluid;wherein the fluid has a pH greater than about 5 and the surfactants havea net negative charge, or the fluid has a pH less than about 9 and thesurfactants have a net positive charge; wherein the one or moresurfactants are selected from the group consisting of a betaine, acarboxylate, a quaternary amine, a quaternary ammonium, a sulfate, asulfonate, a sultaine, and combinations thereof; wherein the one or moresurfactants are attached to the one or more nonmagnetically sensitivenanoparticles forming a S/NSP complex by electrostatic attraction;wherein the S/NSP complex is attached to the one or more magneticallysensitive nanoparticles by electrostatic attraction; wherein at least aportion of the nonmagnetically sensitive nanoparticles are magneticallyaligned; and wherein the nanofluid exhibits an increase of thermalconductivity of at least about 10%.
 12. The nanofluid composition ofclaim 11, wherein the fluid has a dipole moment greater than about zero.13. The nanofluid composition of claim 11, wherein the magneticallysensitive nanoparticles have a positive charge and the surfactants havea net negative charge, or wherein the magnetically sensitivenanoparticles have a negative charge and the surfactants have a netpositive charge.
 14. The nanofluid composition of claim 11, wherein thesurfactants have a pH that imparts or maintains a net negative orpositive charge.
 15. The nanofluid composition of claim 11, wherein thenanofluid exhibits an increase of thermal conductivity of at least about50%.
 16. The nanofluid composition of claim 11, wherein the one or moremagnetically sensitive nanoparticles comprise metals, metal oxides orcombinations thereof; and wherein substantially all of thenonmagnetically sensitive nanoparticles are magnetically aligned.
 17. Ananoparticle composition prepared by the process comprising: combiningone or more nonmagnetically sensitive nanoparticles, one or moremagnetically sensitive nanoparticles, and one or more surfactants;dispersing one or more nonmagnetically sensitive nanoparticles, one ormore magnetically sensitive nanoparticles, and one or more surfactantsin a dispersion; and exposing the dispersion and its contents to amagnetic field, wherein the nanoparticle composition exhibits anincrease of thermal conductivity of at least about 10%; wherein the oneor more surfactants are selected from the group consisting of a cationicsurfactant, an anionic surfactant, a zwitterionic surfactant, andcombinations thereof; wherein the one or more surfactants are attachedto the one or more nonmagnetically sensitive nanoparticles forming aS/NSP complex by electrostatic attraction; wherein the S/NSP complex isattached to the one or more magnetically sensitive nanoparticles byelectrostatic attraction; and wherein at least a portion of thenonmagnetically sensitive nanoparticles are magnetically aligned. 18.The nanoparticle composition of claim 17, wherein the S/NSP complex isformed prior to the dispersing step.
 19. The nanoparticle composition ofclaim 17, wherein the combining step further comprises combining the oneor more nonmagnetically sensitive nanoparticles, one or moremagnetically sensitive nanoparticles, and one or more surfactants in afluid, and wherein the process further comprises attaching one or moremagnetically sensitive particles to one or more S/NSP complexes, one ormore nonmagnetically sensitive nanoparticles and one or moremagnetically sensitive nanoparticles in the fluid.
 20. The nanoparticlecomposition of claim 19, wherein the attaching one or more magneticallysensitive particles to one or more S/NSP complexes occurs prior todispersing the one or more surfactants; or wherein the attaching one ormore magnetically sensitive particles to one or more S/NSP complexesoccurs after dispersing the one or more surfactants.